In wireless communication, e.g., wireless LAN (Local Area Network) and mobile WiMAX (Worldwide Interoperability for Microwave Access), the service of which has launched in recent years, the supply of a communication device including a downsized antenna has been requested by the market. Such a new wireless communication standard tends to allocate frequency band or bands different from one country or region to another. Therefore, the communication device to be supplied to the market is desired to be compatible with all of these different frequency bands. This is because the development of different communication devices in accordance with the frequency bands for the respective countries or regions results in an undesirable increase in cost. In view of this, it is desired to develop a small wide-band antenna usable even in a mobile environment.
Such an antenna is described in, for example, Japanese Laid-open Patent Publication No. 2005-86794 and Yongho Kim, Jun Ito, and Hisashi Morishita, Department of Electrical and Electronic Engineering, The National Defense Academy, “Study and Reduction of Mutual Coupling between Two L-shaped Folded Monopole Antennas for Handset,” IEICE (The Institute of Electronics, Information and Communication Engineers) Transaction on Communication, Mar. 27, 2008.
In WiMAX service, a first frequency band may be allocated for the service in a first country or region, and a second frequency band in a second country or region. At present, for example, a frequency band of 2.5 to 2.7 GHz is allocated for WiMAX service in Japan and a frequency band of 3.4 to 3.6 GHz in Europe. Accordingly a small wide-band antenna and a wireless communication circuit operable in both frequency bands will provide a communication device without replacing the antenna for both bands.
Further, WiMAX employs a MIMO (Multiple Input Multiple Output) communication system. In MIMO, a plurality of transmitting antennas and receiving antennas are provided to simultaneously communicate different communication signal sequences from a plurality of transmitting antennas through channels in the same frequency band, to thereby achieve a substantial increase of efficiency in frequency as a resource.
In this case, if the plurality of antennas are placed in proximity to one another, the mutual coupling thereof is enhanced to result in failure of the realization of the MIMO communication system. Accordingly, it is desired to provide a plurality of antennas contributing to a reduction in required space and weakly coupled to one another.